feat(serve): automatic shared-prefix write at the system/developer frontier

[Astrangemaninhere]

Closes #142

What

Agent clients (Hermes, OpenCode, Pi, etc.) rarely send prompt_cache_breakpoint; on master they always miss the shared system/tools head. This adds one automatic write candidate at the leading system/developer frontier so the existing shared-prefix machinery can publish a prefix those clients actually share.

• Keeps the OpenAI explicit-breakpoint contract and the existing last-content implicit candidate (both writes coexist).
• Opt-out flag --no-auto-system-shared-prefix (extra write on by default) for the Responses path; the Chat path currently uses the default-on policy value (consistent with the issue discussion; wiring the flag into the Chat parse signature was deliberately left out to keep the diff minimal).
• No second store, no new catalog, no engine changes - the planner's existing shared_stable_prefix path does the work.

Per-file rationale (8 files, +65/-7, serve layer only)

1. src/serve/openai_common.h (+5) - OpenAIPromptCachePolicy gains auto_system_shared_prefix (default true). The policy is the single switch that reaches both Chat and Responses without threading a new parameter through every parser.
2. src/serve/openai_common.cpp (+31/-1) - the actual feature:
   • Locates the end of the contiguous leading system/developer run (not the first turn): requests that share both a system and a developer turn reuse the whole instruction prefix.
   • Publishes the DefaultAutomatic candidate on that last leading turn when unmarked (the existing explicit-breakpoint contract is untouched).
   • Reserves one of the four frontend marker slots for the new candidate (frontend.cpp rejects >4 markers); the explicit-write selection shrinks by one when the candidate is distinct, so a request with 3 explicit breakpoints + both automatic writes stays at 4.
3. src/serve/serve_options.h (+3) - ServeOptions gains auto_system_shared_prefix = true.
4. src/serve/serve_options.cpp (+4/-1) - parses --no-auto-system-shared-prefix and documents it in the usage text.
5. src/serve/openai_responses.h (+8/-1) - OpenAIResponsesPromptRequest carries the parsed cache_policy so the policy can be applied after prompt resolution (at parse time the leading turns are still in input_turns/instructions, not in generation.messages).
6. src/serve/openai_responses_request.cpp (+11/-3) - stores the policy on the prompt instead of applying it to the empty message list; the parser signature gains the opt-out bool from the HTTP layer.
7. src/serve/openai_responses_state.cpp (+7) - resolve_openai_responses_prompt applies the policy once generation.messages is assembled (instructions + input turns), which is the earliest point the leading-instruction candidate can be placed correctly. (Fixes the review finding that Responses requests saw no system/developer turns.)
8. src/serve/openai_responses_http.cpp (+3/-1) - passes options_.auto_system_shared_prefix into the parser; without this one line the flag would exist but never reach Responses requests.

Verification

• Build: clean on master da49c0d (WSL2, CUDA 13.3, sm_120a).
• Serve smoke: sibling Chat requests sharing a long system head reuse the published prefix without any client breakpoint; Responses requests place the candidate after the resolved leading instructions.
• --no-auto-system-shared-prefix restores strict OpenAI-implicit behavior on the Responses path.

Follow-ups agreed with the issue author (#142 discussion, not blockers)

• Frontier: if the Qwen template renders tools before the system block, the candidate should sit after the combined tools+system prefix. Open to folding in.
• Floor: skip the extra write below ~256 tokens of shared head.
• @steve8697 offered to re-run the 16k sibling probe against this patch.

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[Astrangemaninhere]

这次的pr修改程度较小，希望能得到采纳；如果不能接受，希望能给出意见。